The purpose of radiation crosslinking for solid polymers is to facilitate air fluidized bed grinding of the subject polymer to a fine powder. Many polymers, including PTFE, can only be ground by using an air jet mill in fluidized bed form. The more uniform the radiation dosage, the faster grinding can proceed. In addition, irradiation is most efficient if the polymer temperature during irradiation can be maintained below a predetermined maximum temperature.
The methods currently in practice to irradiate polymers including but not limited to polytetrafluorethylene (PTFE), are batch processes, which are slow and result in non-uniform irradiation and cooling. Dillon (U.S. Pat. No. 3,766,031) teaches the use of trays containing chips which make multiple passes under a radiation beam. Multiple passes reduce problems of heat and discoloration of the polymer. But despite the use of multiple passes, this tray method is slow and inefficient, and results in some non-uniform irradiation and cooling.
Neuberg and Luniewski (U.S. Pat. Nos. 4,477,192 and 4,748,005) teach the use of a ribbon blender for irradiation. This is also a batch process employing a single charge of chips placed into a water-jacketed ribbon blender wherein agitation for periods up to eight hours is relied upon for surface exposure to the electron beam. The ribbon blender method is slow and inefficient because of time consumed in loading and unloading the blender. Further, because it is common to process recycled PTFE scrap from numerous sources where the specifications, including density, vary widely, the radiation dosage is non-uniformly applied, particularly where the polymer chips are of differing densities and heavier chips tend to seek the bottom of the blender and resist complete agitation. The ribbon blender method also suffers from only random exposure of chips to the cooling surface of the blender shell, and thus, non-uniform cooling.
In short, the prior art methods are slow, chips are not uniformly radiated, and they are not uniformly cooled.